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THE DEVELOPMENT OF A RECORDING RAIL-BOND TESTER
BY
CYRUS WHITE BASSETT
THESIS
FOR
DEGREE OF BACHELOR OF SCIENCE
IN
RAILWAY ELECTRICAL ENGINEERING
COLLEGE OF ENGINEERING
UNIVERSITY OF ILLINOIS
PRESENTED JUNE 1911
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UNIVERSITY OF ILLINOIS
May 25,1911.
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THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Cyrus White B asset t
'LED Development of a Recording Rail-bond Toster.
IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF B.S. in Railway Electrtcal Engineering;,
Instructor in Charge
Approved:
HEAD OF DEPARTMENT OF
197542
UlUC
THE
DEVELOP HE IT T 0 F A RECORDING RAIL-BOHD TESTER
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Digitized by the Internet Archive
in 2013
http://archive.org/details/developnnentofrecOObass
TABLE OF COUTSUTS
Page
Introduction ....... 1
Rail-bond Testing Methods ..... 2
General Description of Principles and Equipment 3
Details of Construction and Operation
Truck Insulation ...... 5
Local Current Circuit ..... 6
Potential Drop Circuit ..... 6
High Tension Circuit ..... 8
Car Speed ........ 9
Recording Mechanism. ..... 10
Record ........ 11
Division of Current in Rails .... 11
Conclusion ........ 12
Diagram of Circuits ...... 13
Sample Section of Record ..... 14
Photographs ........ 15
IITTRODUCTIOK
The early hivstory of electric traction v/as one of rapid, development and expansion. Due to the extreme pressure attendant on the earlier installations little attention was paid to the economies of maintenance and operation. Recently, however, these matters have heen receiving more nearly the attention they deserve. The question of track "bond inspection has "been rather neglected, for about the only means of testing is with a portable milli— voltmeter equipment, hut this method is not at all efficient.
The early electric lines depended upon the earth return from the cars to the station, hut the liability to electrolysis made it imperative to have a good return circuit. The rail itself is an excelle'^ conductor, but the contact resistances of the fish- plates is so great as to amount to practically an open circuit, hence it is necessary to have a bonded joint of good conductivity. In England regulations v/ere passed restricting the drop in the track return circuit to seven volts maximum. In some cities in the United States two overhead contact wires are required.
Rail bonding resolves itself into the problem of maintain- ing a low resistance track return, for when the track bonds are in poor condition a noticeable loss in economy of operation results. It has been noted that the power output required at a sub— station
has been less on a v/et day, ov/ing to the better contact between
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I the rails and the ground.
RAIL-BOIID TESTn^G IvIETHODS
A frequent method of testing rails "bonds is with a portable mill— volt net er equipment, The usual process is to compare the resistance of the bond with the resistance of a certain length of ; solid rail, depending upon the existing current in the rail to give
i a reading. This is a slow and expensive process.
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i Another method for obtaining a general idea as to the con—
I dition of the track return is to measure its total resistance. If the resistance is excessive, each "bond must he inspected separately.
The demand for a less laborious method of testing rail bonds I led to the development of an automatic testing apparatus by A. 3. ! Herriok. A test car was equipped v/ith apparatus for obtaining the
I resistance of the bonds as it passed over them at a moderate speed.
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■ The commercial importance of this test car may be judged from the \ fact that over 90% of the electric lines in Kew England have been I tested.
j The 7/orcester Polytechnic Institute also has a test car
equipped for testing rail bonds, and it is much in demand by the electric lines in that part of the country.
Recognizing the fact thf^t such a testing equipment would have a commercial value, the Electric Railway Department of the University of Illinois undertook to equip their test car with the apparatus necessary to test rail bonds. The following is a des- cription of the construction and testing of such an equipment.
SEKSPAL DSSGPJPTIOU OP PPIi^TCIPLES AWD EQUIHvIEICT
The essential features of this equipment areas follows: A large looal current is circulated "between the two trucks of the I car. The relative resistance of the rail and hond is measured "by I obtaining the voltage drop over that section of track hetween the I trucks. It is necessary to maintain this local current in the
I track in order to ohtain the iDond resistance measurements entirely
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, independant of the normal track current due to other cars on the line.
j The local current was furnished, either by a low voltage
generator or a storage "battery. One of the trucks was insulated from the car body. The local current is conducted from the gen—
I erator and battery through a reversing switch to the car trucks,
I and the oirouit is completed through the rails.
j The voltage drop is obtained by a recording milli— voltmeter
connected to two brushes sliding on the rail. This drop gives the resistance of the rail bond in terras of the resistance of continu— j ous rail. The conductivity of bonds is generally from one— fourth i to one— half that of an equal length of rail. An average value of one— third may be assumed, hence, a rail bond one foot long would have a resistance equivalent to three feet of continuous rail. As the brushes slide over a predetermined length of continuous rail they measure its resistance; when they pass over a joint they measure the resistance of the same length of rail plus the resist- ance of the bond at the joint.
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Two conplete sets of recording instruments should "be pro- vided for measuring "both rails simultaneously. The following description is that of one set only as there was not enough equix>" ment available for two sets.
The voltage leads from the "brushes are connected through a reversing switch and automatic cut out relay to the recording milli— voltmeter. The relay is necessary to protect the mllll- voltmeter from injurious voltages and it may he set to operate at any desired voltage.
The movement of the milli— voltmeter needle is recorded hy puncturing a moving strip of imper with a high tension sparlc. The record paper is positively driven in one direction hy mechanism from one of the car axles through reversing gears. There is a definite ratio between the movement of the cer and that of the paper. The location of poles along the track is indicated b3'- means of a magnetically operated offset pen in conjunction with a con- secutively numbering stamp. Other points are located by means of stamps and by marking the record.
In testing rail bonds it is necessarjr to operate the car at a slow speed. Operating at slow speeds continually with the normal control would overheat the resistance grids. To provide for continuous slow speed the main motor circuit was rev/ired so that the four motors can be connected in series.
DETAILS OF GQI'ISTHUCTIQir KKD OPSRATIOI
Truck Insulation,
The circulation of the large local current
from one truck to the other through the rails makes it necessary to electrically insulate one truck from the car hody. On this type of car it was found necessary to insulate at four places, nanely, the upper center plate, the king pin, the lower side hearings, and the hrake rigging at the radius rod. The voltage hrushes are also insu- lated from the tiruck. The insulating mgiterial throughout is fibre. The details of the insulation are shown by the following drawings:
; >^^^^^^l-l■^^^^^i
Center BtAKinc,. Kinc, Pin ahd Side BEARinr, insuLATiori.
VOLTAQE DROP Bf?U5H. iMSULATIOr*.
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Local Current Circuit .
The lovi tension current for use in the rails is furnished h^ a six cell storage hattery and "by a 100 amp. 3 volt Shunt generator. The capacity of each storage cell is 20 ampere— hours. The generator is "belt driven "by a 4 H.P. 500 volt D.G. Shunt motor. The "battery and generator are connected in the circuit as shown on page 13. The "battery alone furnished the current the greater part of the time. By means of a galvanized iron rheostat the current from the hattery can "be varied from 5C to 250 amperes.
The reverse switch is used to send the local current through the rails in the same direction as that of the normal track current. The current used in operating the car is also added to the local current by means of a dou"ble throw switch con- necting the motor ground wire to either truck.
Potential Drop Circuit .
The function of this circuit is to o"btain the voltage drop from the rail "by means of two steel wire brushes and convey it to the recording milli— voltmeter . The brush- es were most conveniently mounted in guides bolted to the inner end bar on each truck, thus making the distance between brushes equal to 12 feet. The brushes move vertically in the guides, thus allo^v- ing them to follow the track closely, and are weighted to give the necessary pressure to assure good contact with the rail. In round- ing curves the brushes do not leave the track as they were mounted quite close to the wheels. The brushes were made by filling the heads of steel wire roughing brushes with babbit and embedding a
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copper lead therein. Each "brush is insulated from the truck.
On A. 3. Herrick's test car the distance hetv/een "brushes was 4 feet, while here a distance of twelve feet gave satisfactory results. This spacing has the advantage of perraitting a higher maintained speed than the shorter spacing would allow for o"btaining satisfactory nilli-voltneter readings.
The voltage drop current from the two "brushes is led to a reversing switch; from there it separates into two circuits, one going through an automatic cut— out relay and the other to the recording milli— voltmeter through a contact on the armature of the relay.
The relay is designed to operate when the voltage drop across its terminals exceeds a certain predetermined amount. It is required to operate on a very lov; voltage. Hence the magnet coils are wound with a comparatively few nura"ber of turns of rather large cross— section wire. It was found necessary to construct the entire relay, and in this case each of the two coils was wound with 240 turns of ^To. 14 D.G.C. v/ire. The core is of soft iron wire to insure quick action of the armature. The tension on the armature is regulated "by a spring.
The circuit to the milli— voltmeter is completed through a contact on the relajr armature. An excessive voltage drop causes the armature to "be attracted; instantly opening the contact and cutting the milli-voltmeter out of the circuit.
To properly complete the function of the relay the armature when attracted should close a circuit actuating an offset pen for marking "open" or high resistance "bonds.
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The construction of this relay was not all that could "be
desired, as the means were not at hand to mal^e so delicate an
instrument. However, it would promptly cut out the milli-voltmeter
on voltages exceeding 200 milli— volts.
High Tension Circuit .
For all voltage drops, up to and including 75 milli— volts, the movement of the milli— voltmeter needle is recorded hy means of a high tension spark. The needle is insulated from the milli— voltmeter movement hy glass tuhing. The high tension spark is hrought from the induction coil to a copper strip placed just oelow the needle and ahout one inch from the tip. The spark jumps from the copper strip to the aluminum pointed needle, passes to the tip, and from there jumps to the end of the nearest one of a nuraher of copper wires insulated from each other, and extending in the form of an arc just under the tip of the needle throughout its whole travel. The copper wires terminate in a straight row in a "block of fihre placed slightly ahove the record paper. Thus as the needle sv/ings the spark travels hack and forth, and issues from the wires in the fihre hlock. The spark jumps to a hrass plate over which the record paper is drawn and thence returns to the induction coil through a wire grounded on the hrass plate. The spark in its passage from the wire ends to the hrass plate punctures the record, thus giving an indication of the needle ;which in turn indicates the voltage drop.
A Max Zohl induction coil is used. The primary current is obtained from a storage battery and is controlled hy a rheostat. The spark from the secondary gives a clearly punctured record.
To provide for either normal car operation or for the slower continuous speed required v:hen testin£y rail "bonds, necessitated the rev/iring of the main aotor circuit. Thus opening switches 1, 3, 4 and 6, and closing 2 and 5, (see diagram) puts the four notors in series on the line. This arrangement gives a continuous running speed of, approximately one— fourth full speed, or IE M.P.H. By closing either switch 7 or B, the main car current could ho sent to either truck to add to the local circulating current in the track.
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jmsb-t
L- O —
_ 7 & _
T;?ucK no I. Ti?ucK Mo a.
While a speed of IS M.P.H. is found to give satisfactory- results with a twelve foot spacing "between the voltage hrushes, if it were found desirable to decrease the spacing in order to secure greater sensitiveness, a lower speed would he necessary to give sufficient tine for the needle to record the full drop across the hond. However, the twelve foot spacing is hetter than a shorter spacing for use on poorly honded tracks, as it gives a large range and great accuracy is not required.
P.ecording Kechanisrn.
The record is made on a continu-
ous strip of paper 10 inches v;ide. It is positively moved in one direction irrespective of the car moveaent. The paper cones from a roller under the table, up through tho table and over the brass plate bridge; then down through the table to two driving rollers and finally is rewound upon the receiving roll. The driving rol- lers are operated from the car axle by means of gears and flexible shafting. The gear ratio is such that 90 feet of car travel moves the paper through a distance of one inch. The driving mechanism is shown by the following drawing:
Flexible ShAmuG
DRivific, Rollers
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The Record.
Page 14 is a sample section fron a record made on the road. It is a record of one rail only. The trollejr poles are indicated b:/ the offsets in the line, and the pole offsets are stamped with numbers, starting from some one pole v/hose position is easily located. Hence the position of any high resistance "bond may "be determined by reference to the pole offsets and the corresponding niimber which is referred to some starting point. Other locating points, as crossings, curves, sidings, stations, etc. may be marked with pencil as passed.
The ordinate R represents the voltage drop on 12 feet of* continuous rail, while T represents the voltage drop over 12 feet of rail and the included bond. x 12 gives the equivalent
resistance of the bond in terms of feet of continuous rail. This is based on the assumption that the current in the rail remains constant .
Division of Current in Rails.
The sample test record shows that the deflection, R, on continuous rail and, therefore, the current in this rail remains practically constant as long as a bond is not included in the circuit. However, it is evident that the introduction of a high resistance bond into the circuit will decrease the current. The amount of decrease in the current depends upon the ratio of the bond resistance to the total resistance of the. circuit between the truclrs, which includes the contact resist- ances between the wheels and rails, and also the rail resistance and the bond resistance. This ratio is very small as was shovm by
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the fact that during tests the voltage between truclrs remained fairly constant at 1.2 volts, v/ith a local iDattery current of 120 amperes, and 40 amperes from the main motors flowing from one of the trucks into and through the rails in the same direction as the hattery current.
Tests made with the car standing show that the greater part of this drop is due to the contact resistance hetv/een the wheels and rails. There is a certain amount of drop due to the inductance in the wheels and rails owing to the fact that this magnetic cir- cuit through which the current flows is continually changing as the car advances.
Both the contact resistance and the inductive effect tend to prevent the unequal division of current in the rails.
The full scale deflection of the recording mi Hi— voltmeter is only 75 milli-volts, and it takes a high resistance hond to give this deflection. Ilence the hond resistance is never great enough, when compared with the contact resistance between the wheels and rails, and the inductance, to materially affect the division of cur- rent between the two rails.
Conclusion.
In the few tests that have been made since the completion of the apparatu.s, it has been impossible to obtain accurate measurements of the resistance of bonds. However, the records made are of commercial value in that they show the relative condition of different sections of the track and indicate which bonds are in the most urgent need of attention.
Rail. Bono Record. iLLliiOis TRACTron 5ystem Oqdem to Gi cover. Test on South f?Aiu. Poles */)06 J0*I0JS Local CuRRErtT. 4o flnp. riAY IS, »9»l.
ij»»vf Rsny Of »L'
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